Conventionally, as one means of connecting circuit members with each other, connecting material tapes (e.g., anisotropic conductive films (ACFs)) have been used, which are formed by coating a release film with a thermoplastic resin containing conductive particles dispersed therein.
The anisotropic conductive films are used for adhering and electrically connecting various terminals with each other, for example, for connecting flexible printed circuits (FPCs) and/or terminals of IC chips with indium tin oxide (ITO) electrodes on glass substrates of LCD panels.
One practically used anisotropic conductive film is cationic-curable anisotropic conductive films each containing a cationic curing agent and an epoxy resin. The cationic-curable anisotropic conductive films realize curability at low temperatures and reduction of warpage of an adherend.
However, sulfonium salts, etc., serving as the cationic curing agent, have high curing activity and thus, the curing reaction is easily inhibited by, for example, even a trace amount of impurities, causing problematic phenomena such as poor curing.
In particular, such poor curing often occurs due to a passivation film of polyimide formed on the back surface of an IC chip. Specifically, in the case where an IC chip is connected via a cationic-curable anisotropic conductive film, at the time when the cationic-curable anisotropic conductive film is attached to the IC chip to initiate curing reaction, the generated cationic species (H+) are inactivated by polyimide materials of the passivation film. One possible reason why the generated cationic species (H+) are inactivated by the polyimide materials of the passivation film lies in that the cationic species (H+) react with nitrogen atoms (N) contained in polyimide and are then trapped (the reaction R3N→R3N+H occurs to form ammonium salts).
In addition, even when an IC chip is connected using a TAB tape formed by laminating a polyimide film on a Cu foil with an adhesive, the adhesive is made of polyamide and thus, causes inhibition of curing.
Another practically used anisotropic conductive film is radical-curable anisotropic conductive films each containing a radical curing agent (an organic peroxide) and an acrylic resin. The radical-curable anisotropic conductive films are often employed for connection at the PWB side, and realize satisfactory curability at low temperatures. However, the radical-curable anisotropic conductive films do not generate a hydroxyl group during curing and thus, the interaction between the film and the adherend having polarity becomes weak, causing problematic phenomena such as poor curing. In particular, the radical-curable anisotropic conductive film has poor adhesion to the glass surface at the LCD panel side, problematically causing peeling at the interface. The radical-curable anisotropic conductive film, therefore, is not suitable for connection at the LCD panel side, and also is not used widely.
In addition, a rubber material commonly used as the material for an anisotropic conductive film is easy to cause inhibition of curing, and use of the rubber material as a stress releaser after curing is limited. Also, the anisotropic conductive film made of the rubber material tends to be high in elastic modulus and Tg, easily causing strain of a base after curing.
Further, there is proposed a dual-curable anisotropic conductive film containing a radical curing agent (for curing at low temperatures) and an imidazole curing agent (for curing at high temperatures) (e.g., Japanese Application Laid-Open (JP-A) No. 2007-262412). However, an anisotropic conductive film formed from components having different curing mechanisms tends to cause phase separation during curing and to involve inner cracks, resulting in degraded connection reliability. Also, curing must be performed at two steps, which is not suitable for connection in a short time.
Furthermore, proposed are a dual-curable anisotropic conductive film formed from a radical curing agent and a cationic curing agent (e.g., JP-A No. 2006-127776); an anisotropic conductive film containing, in a binder, a heat-curable composition and a photo-curable composition (e.g., JP-A No. 2005-235956); and a two-layered anisotropic conductive film having a photo-cationic curing agent-containing layer and a photo-radical curing agent-containing layer (e.g., International Publication No. WO00/046315 pamphlet). Neither of them cannot improve curing failure by a passivation film of polyimide formed on the back surface of an IC chip. Thus, demand has arisen for development of an anisotropic conductive film which involves no curing failure by a passivation film of polyimide.
Further, in the recent display device members such as LCDs, PDPs and organic ELs, in use, wires of metal such as Al, Mo, Cr, Ti, Cu or Ni are often formed on a base of ITO from the viewpoint of, for example, electrical conductivity. Thus, it is difficult to connect circuit members with each other by photo-curing an anisotropic conductive film placed therebetween.